1. Field of the Invention
The invention relates to page-wise storage systems, in particular holographic storage systems.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Holographic systems typically involve the storage and readout of entire pages of information, these pages consisting of arrayed patterns representing information. In general, a holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, November 1995.
Holographic systems are characterized by their potential for both high density storage potential and high retrieval speed. In fact, because information is typically manipulated, i.e., stored and retrieved, on a page-by-page basis, the speed of storage and retrieval compares favorably to conventional magnetic disk or compact disk storage systems. A significant advantage of holographic systems, however, is storage capacity. It is possible for each page stored as a holographic image to contain thousands or even millions of elements. Theoretically, it is believed that at the present time, up to 1014 bits of information are storable in approximately 1.0 cm3 of holographic storage medium.
FIG. 1 illustrates the basic components of a holographic system 10. System 10 contains a spatial light modulator (SLM) 12, a photorecording medium 14, and a sensor 16. The SLM 12 is any device capable of optically representing data in two-dimensions. The SLM 12 is generally attached to an encoding unit which encodes data onto the modulator. Based on the encoding, the SLM 12 selectively passes or blocks portions of a beam passing through it or reflecting off of it. In this manner, the beam 20 is encoded with a data image. The image is stored by interfering the encoded signal beam 20 with a reference beam 22 at a location on or within photorecording medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22 (generally referred to as angle, wavelength, and phase correlation multiplexing, respectively). Signal beam 20 typically passes through lens 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lens 32 before this intersection.
Once data is stored in medium 14, it is possible to retrieve the data by intersecting reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase at which reference beam 22 was directed during storage of the data. The reconstructed data passes through lens 34 and is detected by sensor 16. Sensor 16 is, for example, a charged coupled device or an active pixel sensor. Sensor 16 typically is attached to a unit that decodes the data.
Unfortunately, while page-wise storage and readout in a holographic system offers the potential for high speed and capacity, the page-wise nature of the system also introduces potential problems. For example, each pixel generated by the SLM 12 has a corresponding target pixel on the sensor 16, i.e., the system is designed such that each pixel stored in the medium by directing the signal beam through the SLM will, upon readout, be directed to a particular, corresponding pixel on the sensor. If, at any point in the system, the page of information is offset by a single pixel or the image becomes blurred or distorted, the stored information will be unreadable. Introduction of non-imaging elements into the system, e.g., storage media, tend to induce aberrations in the relayed image, thereby contributing to this problem. In particular, while putting the medium at an angle to the signal beam is generally desirable for photopolymer media and for some multiplexing geometries, such a configuration tends to introduce even more significant aberration.
It is therefore desired to develop techniques to reduce or avoid the introduction of aberrations into holographic systems, particular aberration introduced by non-imaging elements such as storage media.
The invention provides a process and apparatus for holography in which aberrations introduced by storage media, particularly photopolymer-type media, are substantially reduced. Specifically, as reflected in FIG. 3, photopolymer-type media 56 are generally placed at an angle to the optical path 64 in a holographic system, in order to reduce the effects of writing-induced shrinkage. This angular placement, however, introduces aberrations into the relayed image of the system, including astigmatic aberration, i.e., a tilt in the plane of focus 62. This tilt causes the page of data to match up poorly to the plane of the sensor 60, thereby affecting the accuracy of read-out.
It has been discovered, however, that it is possible to compensate for at least a portion of such aberration by introducing an orthogonal tilt in the spatial light modulator, as shown in FIG. 4. The rotation of the SLM 70 creates a similar aberration in the plane orthogonal to the plane tilted by the medium, and thereby makes the data page come into a sharper focus over the entire page. This effect allows presentation of a flatter, more focused, and less distorted image 80 at the sensor 82, and leads to better matching of each encoded pixel to its target pixel.
The invention therefore involves a holographic system containing (a) a storage medium (74) located in the system""s optical path (84), where the medium is rotated around a first axis (76) such that the surface of the medium is in a non-orthogonal relationship with the optical path, and (b) a spatial light modulator (70) also located in the optical path, where the modulator is rotated around a second axis (72) that is substantially orthogonal to the first axis, and wherein the surface of the modulator is in a non-orthogonal relationship with the optical path. (Optical path indicates a path from the spatial light modulator through the medium and onto the sensor, and is also referred to in the art as the central ray.) The invention is particularly useful for 4F imaging systems.